This application claims priority from Japanese Application No. 10-071606 filed Mar. 20, 1998, the entire content of which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a liquid crystal display device and, in particular, to a stack-type liquid crystal display device having stacked liquid crystal layers.
2. Discussion of the Background
Being thin and of low power consumption, liquid crystal display devices are now widely used in notebook-sized personal computers. In particular, the low power consumption is a feature that makes liquid crystal display devices superior to other displays such as CRT displays and plasma displays. Liquid crystal display devices are expected to be increasingly widely applied to personal information equipment in the future.
In portable equipment, it is desirable that the power consumption of the display be 500 mW or less, preferably as low as several milliwatts. Consequently, the reflection-type liquid crystal display device is desirable because it does not require a backlight and is of low power consumption. Among reflection-type color liquid crystal display devices are the in-plane type devices using color filters. However, if the color purity in these devices is improved, the light utilization efficiency is lowered by a factor of three or more, and hence the reflectance is decreased.
In view of the above, stack/reflection-type liquid crystal display devices having stacked liquid crystal layers have been proposed (refer to Japanese Unexamined Patent Publication No. Hei. 8-313939, for example). In the reflection-type liquid crystal display device disclosed in the publication No. Hei. 8-313939, guest-host liquid crystal layers of cyan, magenta, and yellow are used. To apply potential differences to the respective liquid crystal layers, pixel electrodes are provided so that each liquid crystal layer is interposed in between and an active matrix substrate is provided under each liquid crystal layer. The pixel electrodes are connected to respective active elements such as TFTs via columnar electrodes. A desired display image is produced by applying prescribed potential differences to the respective liquid crystal layers in the form of potential differences between the pixel electrodes. Although this type of stack-type liquid crystal display device enables high light utilization efficiency and hence can provide a bright reflection image, it requires the application of differential potential differences to the respective liquid crystal layers in the driving of the pixels.
Japanese Unexamined Patent Publication No. Hei. 9-80488 has proposed one method of applying differential potential differences, in which differential potential differences are generated in a pixel by driving pixel layers (sub-pixels) in a time-divisional manner (time-divisional differential driving) and rendering the other pixel layers in a floating state when a potential difference is applied to one pixel layer. However, the potentials of pixel electrodes that are rendered in a floating state tend to vary due to coupling with scanning lines, signal lines, or the like, possibly resulting in deterioration in image quality such as crosstalk.
Further, as for the use of auxiliary capacitors in the time-divisional differential driving, sufficient studies have not been made of how to connect and arrange the auxiliary capacitors to effectively reduce the degree of coupling with scanning lines and signal lines.
As described above, in conventional liquid crystal display devices that display by applying prescribed potential differences to the respective stacked pixel layers (sub-pixels) in a time-divisional manner, effective measures have not been taken in terms of the driving technique, the layout of auxiliary capacitors. and other techniques in order to reduce the degree of coupling with scanning lines and signal lines that are provided on the lower-layer side.
The present invention has been made in view of the above problems in the art. Therefore, in a liquid crystal display device that displays by applying prescribed potential differences to respective stacked pixel layers in a time-divisional manner, an object of the present invention is to reduce the degree of coupling with scanning lines and signal lines provided on the lower-layer side. In other words, an object of the present invention is to shield the liquid crystal layers from scanning and signal lines.
A liquid crystal display device according to the present invention comprises a plurality of pixel electrodes and a plurality of liquid crystal layers that are stacked alternately, a shield electrode that is disposed below the lowest pixel electrode, and a circuit for supplying the shield electrode with a potential corresponding to a potential that is supplied to the uppermost pixel electrode.
The uppermost and lowest pixel electrodes are the uppermost and lowest ones of the stacked pixel electrodes that function substantially to apply prescribed potential differences to the respective liquid crystal layers.
The circuit for supplying the shield electrode with a potential corresponding to a potential that is supplied to the uppermost pixel electrode may short circuit the shield electrode to the uppermost pixel electrode.
In the liquid crystal display device of the present invention, after prescribed potentials have been applied to the respective liquid crystal layers other than the uppermost liquid crystal layer by supplying potentials to the respective pixel electrodes, the pixel electrodes that have served to apply the prescribed potential differences are rendered in a floating state, and prescribed potentials are applied to the uppermost pixel electrode and the shield electrode.
It is preferable to provide a common electrode in such a manner that the uppermost liquid crystal layer is interposed between the uppermost pixel electrode and the common electrode (the uppermost liquid crystal layer is disposed above the uppermost pixel electrode). In this case, a prescribed potential difference may be applied to the uppermost liquid crystal layer by supplying prescribed potentials to the uppermost pixel electrode and the common electrode.
In the above liquid crystal display device, after potential differences corresponding to display signals have been applied to the liquid crystal layers other than the uppermost liquid crystal layer, the liquid crystal layers other than the uppermost liquid crystal layer are rendered in a floating state and a potential difference corresponding to a display signal is applied to the uppermost liquid crystal layer. At this time, the potential of the shield electrode that is disposed below the lowest pixel electrode varies in link with the potential of the uppermost pixel electrode. Therefore, even if regions exist below the shield electrode that are given prescribed potentials such as signal lines, scanning lines, and active elements, potential variations at the pixel electrodes due to coupling with those regions can be inhibited. Further, the potential differences applied to the respective liquid crystal layers can be maintained.
Another aspect of the present invention provides a liquid crystal display device having a shield electrode that is disposed below the lowest pixel electrode in which after prescribed potentials have been applied to the respective liquid crystal layers other than the lowest liquid crystal layer, the pixel electrodes other than the lowest pixel electrode are rendered in a floating state, and a prescribed potential difference is applied to the lowest liquid crystal layer by supplying proper potentials to the lowest pixel electrode and the shield electrode. In this liquid crystal display device, after potential differences corresponding to display signals have been applied to the liquid crystal layers other than the lowest liquid crystal layer, the liquid crystal layers other than the lowest liquid crystal layer are rendered in a floating state and a potential difference corresponding to a display signal is applied to the lowest liquid crystal layer. At this time, the shield electrode that is disposed below the lowest pixel electrode has a prescribed potential. Therefore, even if regions exist below the shield electrode that are given prescribed potentials such as signal lines, scanning lines, and active elements, potential variations at the pixel electrodes due to coupling with those regions can be inhibited. Therefore, the potential differences applied to the respective liquid crystal layers can be maintained. It is preferable that the shield electrode be a common electrode of the lowest liquid crystal layer.
Another aspect of the present invention provides a liquid crystal display device comprising a plurality of pixel electrodes and a plurality of liquid crystal layers that are stacked alternately, and a plurality of electrodes for auxiliary capacitors that are disposed below the lowest liquid crystal layer. The auxiliary capacitor electrodes can be stacked in the order that is opposite to the stack order of the corresponding pixel electrodes.
The lowest pixel electrode and the uppermost auxiliary capacitor electrode may be commonized with each other; that is, a single electrode may be provided that has the functions of those two electrodes.
Still another aspect of the present invention provides a liquid crystal display device having auxiliary capacitor electrodes below the lowest liquid crystal layer in which after prescribed potentials have been applied to the respective liquid crystal layers other than the uppermost liquid crystal layer, the pixel electrodes other than the uppermost pixel electrode are rendered in a floating state, and proper potentials are applied to the uppermost pixel electrode and the corresponding auxiliary capacitor electrode.
It is preferable to provide a common electrode in such a manner that the uppermost liquid crystal layer is interposed between the uppermost pixel electrode and the common electrode (the uppermost liquid crystal layer is disposed above the uppermost pixel electrode). In this case, a prescribed potential difference may be applied to the uppermost liquid crystal layer by supplying prescribed potentials to the uppermost pixel electrode and the common electrode.
A further aspect of the present invention provides a liquid crystal display device having a plurality of auxiliary capacitor electrodes that are disposed below the lowest liquid crystal layer in which after prescribed potentials have been applied to the respective liquid crystal layers other than the lowest liquid crystal layer, the pixel electrodes other than the lowest pixel electrode are rendered in a floating state, and a prescribed potential difference is applied to the lowest liquid crystal layer by supplying proper potentials to the lowest pixel electrode and the corresponding auxiliary capacitor electrode.
It is preferable to provide a common electrode in such a manner that the lowest liquid crystal layer is interposed between the lowest pixel electrode and the common electrode (the lowest liquid crystal layer is disposed below the lowest pixel electrode). In this case, a prescribed potential difference may be applied to the lowest liquid crystal layer by supplying prescribed potentials to the lowest pixel electrode and the common electrode.
The auxiliary capacitor electrodes function as shields. Therefore, even if there exist, below the lowest auxiliary capacitor electrode, regions that are given prescribed potentials such as signal lines, scanning lines, and active elements, potential variations at the pixel electrodes due to coupling with those regions can be inhibited. Therefore, the potential differences applied to the respective liquid crystal layers can be maintained.
Since a lower pixel electrode is more influenced through coupling with underlying regions, a higher auxiliary capacitor electrode has a stronger shield effect for the pixel electrodes. If the auxiliary capacitor electrodes are stacked in the order that is opposite to the stack order of the corresponding pixel electrodes, the lowest pixel electrode that is influenced through the coupling most and the uppermost auxiliary capacitor electrode that has the strongest shield effect can be given the same potential. Therefore, the coupling can be minimized.